12.1 Earth, Moon and Sun Interactions

Early civilizations were very interested in astronomy.

Daily and seasonal changes in the sky were observed and used for navigation, agriculture, hunting, and charting the passage of time

Several highly sophisticated structures were designed and built around the world to observe and track celestial movements

History of Astronomy

Early Observatories

Geocentric Model: Earth is the center of the universe

Based on the work of the ancient Greek philosopher Ptolemy

Heliocentric model: Sun is the center of the solar system, and the universe

expands outward.

Based on the observations of Copernicus and Galileo

Kepler’s proposal that planets travel in elliptical orbits brought the model to widespread acceptance

History of Astronomy

Moons are natural satellites orbiting a planet

Earth has one moon

How did it form? Giant Impact Theory

Moon formed as a result of a Mars sized planet hitting Earth a short time after it’s formation

Debris ejected from the collision was trapped in orbit and combined to form the moon

Evidence: Earths crust is very similar in composition to the moon

The Moon

The Moon has no atmosphere to protect it from bombardment by comets and meteors

No erosional forces like wind and water either, which mean the craters appear unchanged

Most impacts occurred 4.1 to 3.8 billion years ago

Light areas are oldest rock types

Dark areas are called mare, made of igneous rock called basalt

The Moon

Moonlight is actually light from the Sun reflected off the Moon’s surface.

As the Moon revolves around Earth the angle we view changes resulting only a portion of the moon reflecting light – Phases

The Moon rotates at the same rate it revolves so the same side is always facing Earth

It takes the Moon 29.5 days to revolve around the Earth – Lunar month

Phases of the Moon

New Moon – Sun is lighting the part of the moon not visible to Earth

Waxing Moon – Visible portion is growing

Full Moon – Entire face of the Moon is illuminated

Waning Moon – Visible portion is shrinking

Phases of the Moon

Whole lunar cycle takes 29.5 days –not quite in sync with our monthly calendar

Blue Moon – Second full moon in a calendar month occur every 33 months.

(next one is in July 2015)

Once in a Blue Moon…

The Earth, Moon and Sun all exert gravitational forces on each other

The Moon being much closer than the Sun exerts a gravitational pull about 2 times as strongly

This pull causes a bulge of water in line with the Moon’s position

As the Moon orbits Earth the location of the bulge follows causing high and low tides

Tides

Spring and Neap Tides

Rotation from west to east day and night

Earth is tilted 23.5° on its axis

Because of the tilt the Sun’s rays hit the Earth at different angles as it travels in orbit causing seasons

Earth’s Rotation and Tilt

The tilt of the Earth causes variation in the day length (hours of sunlight)

Summer Solstice: Sun’s rays hit the Northern hemisphere most directly and we experience the longest day – June 21

Winter Solstice: Sun’s rays hit the Northern Hemisphere most indirectly and we experience the shortest day – December 21

Equinox: 12 hr sunlight, 12 hr darkness

Solstices

Eclipse: The total or partial overshadowing of one celestial body by another

Solar Eclipse: Moon blocks the light from the sun and it’s shadow falls across the Earth

Total Eclipse: Central darkest shadow (Umbra)

Partial Eclipse: Shaded partial shadow (Penumbra)

Eclipses

Lunar Eclipse: Earth lies between the Sun and Moon and it’s shadow falls on the Moon

Eclipses

Why do we not experience a lunar eclipse every month?